This invention relates to fluidized beds and particularly to apparatus for distributing a fluidizing gas to the bed. A typical fluidized bed such as that utilized in a fluidized catalytic cracker unit for example contains a large quantity of relatively expensive catalyst particles which might range in size from about 20-150 microns. In order to minimize the quantity and cost of catalyst required to achieve a given result in a process operation the gas which is directed upwardly through the bed should contact all of the catalyst uniformly and should remain in contact with the catalyst for a maximum length of time. A catalyst unit can be very large, such as 10 meters in diameter. One common way of achieving gas distribution is such a unit is to form a perforated plate under the fluidized bed or a planar assembly of pipes in the bottom of the unit which have myriad branches and perforations therein through which gas is carried to the bed. The pipe type units are expensive while either type of unit is somewhat inefficient since the flow tends to channel upwardly through portions of the bed immediately above the perforations while bypassing other portions. Also, when the blowers which force the gas through the bed are shut down the bed slumps and many of the particles forming it tend to get sucked into the perforations by the back surge of gas which occurs at shutdown. The particles can damage the blowers and can also present problems when the blowers are restarted due to pressure required to force the particles back through the perforations. In addition, and of greater concern, the particles are usually very erosive and their continued contact with the perforations as they pass through them can quickly enlarge the openings. The enlargement will vary the pressure drop of the system and change the flow characteristics so as to affect the efficiency of contact of the catalyst and gas since it is important that a given pressure drop be maintained through the bed.
One type of flow distributor which is in use and intended to reduce backflow of catalyst has a threaded hollow base portion which is integral with an upper radially perforated portion having very large perforations (about 4.76 mm). The upper portion is covered by a conically topped cylinder which causes the gas exiting the perforations to be directed downwardly toward the base in a ring-shaped configuration. The downward flow helps insure maximum contact time with the catalyst since the flow reverses at the bottom of the bed and moves upwardly through the entire bed. However, the aforementioned flow distributors are very expensive. Also, the downward flow of gas tends to blow the erosive catalyst particles against whatever is under the distributor and quickly erode it. The damage can be reduced by placing large washers under the distributors which can be periodically replaced as they become worn.